The field of art to which the invention pertains is liquid-liquid mass transfer between immiscible liquids, for example the removal of acidic compounds carried in a hydrocarbon by contacting the hydrocarbon with an aqueous solution of caustic.
One conventional way to transfer a component of one immiscible liquid into another is the dispersion-settling process in which one liquid is dispersed into the other to form an emulsion and then the emulsion is allowed to settle and separate. One situation in which this dispersion-settling process is used is in the separation of acidic constituents from cracked gasoline. This gasoline is treated with an aqueous caustic solution, which is immiscible in the gasoline, to remove the acidic matter. Conventionally, this treatment consists of dispersing the aqueous caustic solution in the gasoline to form a caustic-in-gasoline emulsion. The acidic constituents in the hydrocarbon phase present at the interfacial boundary between the droplets of aqueous caustic and the surrounding hydrocarbon phase will react with the caustic to form sodium salts. These salts greatly favor the aqueous caustic solution and will enter the aqueous phase and diffusively migrate into the droplets away from the interfacial boundary. After a sufficient time has passed for the acidic constituents to move from the hydrocarbons into the aqueous caustic, the emulsion of hydrocarbon and caustic is placed in a settling tank and the caustic allowed to settle to the bottom.
The efficiency of the mass transfer of the acidic constituents for a given volume of aqueous caustic is improved by (a) increasing the interfacial boundary and (b) decreasing the distance through the hydrocarbon phase that the acidic constituents must diffuse to reach this boundary. This can be accomplished by making the droplets very small, but this increases the energy requirements to create the emulsion. Also, when the droplets are made smaller, the time for them to settle in the settling tank is increased.
Another type of liquid-liquid mass transfer is described in U.S. Pat. No. 3,351,434 issued Nov. 7, 1967 to P. G. Grimes, et al, for a Liquid-Liquid Two-Phase Contactor in which discs are rotated by a motor between two immiscible liquid layers to extract a component from one liquid into the other. Such a system reduces considerably the problems of emulsion settling, but it is still not entirely satisfactory.
The present development creates a large interfacial boundary with very low energy requirements and eliminates the need of waiting for an emulsion to separate.